School districts around here try to get rid of buses at around 15-17 years. This bus would have been dumped at 28 years. I question the claim, and thus question also the claim of the rebuild. Does he have receipts?

And 3.5 million miles? If he wants $500 and the tires are good you might take a gamble with a mechanic's visit. At worst, if it would limp to the scrapyard you could sell them the steel, and keep and resell the tires and make a profit. But otherwise? I'm worried.

I think you've made a decimal point mistake on your mileage. If it is more reasonably 350K, instead of 3500k, it is nearly double the mileage limits most of us look for in a bus. Even if the engine was replaced the rest of the bus has a significantly high amount of miles.
Nobody can judge this bus as accurately as you can. None of us can look at it unless you provide pictures. Think used car. Was this bus only driven to church by a sweet old lady on Sundays?

if the engine was replaced wit ha remanufactured engine like a Jasper or such, you can get the tag number from the engine, callthem and at least get the age of it.. you cant get the accurate miles, however the age may give you some idea of if its an older or newer replacement.
-Christopher

Google GM 6.5 and read some of the problems they've had. Cracked blocks, broken webbing, cracked cylinder walls. The good side, they generally won't "nickel and dime" you as they're more prone to catastrophic failures.

Oddly enough, the turbo versions make pretty decent power until they die.

350k is a rare sight for that motor, and it means the rest of the chassis still has 350k on it.